A new generation detector for supersymmetric particles search by direct detection : MACHe3

HAL Id: in2p3-00020396

http://hal.in2p3.fr/in2p3-00020396 Submitted on 30 Jan 2004

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A new generation detector for supersymmetric particles search by direct detection : MACHe3

Emmanuel Moulin, D. Santos, G. Perrin, G. Duhamel, F. Naraghi, Yu.M. Bunkov, H. Godfrin

To cite this version:

A new generation detector for

supersymmetric particles search

by direct detection :

MACHe3

,

MA

trix of

C

ells of

superfluid

He

lium

3

ISN-CRTBT collaboration:

•ISN : D. SANTOS, G. PERRIN, G. DUHAMEL, F. NARAGHI, E. MOULIN

•CRTBT : Yu. M. BUNKOV, H. GODFRIN Emmanuel MOULIN

Main topics :

¾ Cosmological evidence for cold non-baryonic dark matter

¾ WIMP candidate : the neutralino

¾ MACHe3 project : Î detection principle Î detection threshold Î events ¾ Results : Î neutrons spectrum

Î simulation of muons (with GEANT3.21)

Î muons spectrum

Evidence for

non-baryonic

dark matter

:

WIMPs candidate for

cold non-baryonic dark matter

:

the neutralino

χ

➘ _{cold non-baryonic dark matter is favoured} (structures formation in the Universe)

➘ characteristics of WIMPs ( Weakly Interactive

Massive Particles ) :

• masses : from 30 GeV/c2 _{up to few TeV/c}2 • weak cross section : < 10-2 _pb

• neutral of charge and color

➘ SUSY ⇒ lightest supersymmetric particle (LSP) : the neutralino χ (with R-parity conserved)

• belongs to WIMPs’ class

MACHe3 project for neutralinos

χ

detection from galactic halo :

• Measure of energy deposited by elastic scattering of χ on 3_{He target nucleus}

• 3_{He as sensitive medium :}

- superfluid 3_{He : T ~ 100} µK

- very low energy gap of quasiparticles :

threshold of 1 keV

⇒ ability of detecting weak recoil energies • Main contributions to the background :

¾ gammas, neutrons, muons

¾ protons, α particles (negligible) ¾ micro-vibrations

• Main interesting features : ¾ high purity 3_He

¾ spin 1/2 (axial interaction) ¾ neutron capture process

CRTBT experimental hall :

• cryostat • 5 cm thick

lead shield

Bolometer cell :

• Operating mode : Lancaster type bolometer

damping effect on the vibrating wire of the

quasiparticles cloud produced by an incoming particle interacting inside the cell

• Measure : damping linked to the frequency width

χ event in the granular detector :

➘ _{Elastic scattering on} 3_{He nucleus :}

⇒ χ event defined by an energy deposit ≤ 6 keV

➘ _{The whole energy is deposited in a single cell} ⇒ systematic discrimination compared with other

events

➘ Expected event rate :

R = 1400 × σ (pb) / Mχ (GeV/c2) [kg−1 _day-1]

With σ ∼ 10−2 _{pb and Mχ ∼ 30 GeV/c}2

⇒ R ~ 7 × 10-2 _kg−1 _day-1

(

)

➘ _{2 contributions to the elastic scattering on nucleon :}

• scalar interaction : ~ 10-6 _pb

➘ low frequency modulation ➘ _{micro-vibrations}

➘ very low micro-vibrations

Raw data at 100

µΚ

:

Sensitivity of the cell :

Acquisition spectrum at 100 µK, without source : ➘ _{3 peaks of about 10 keV}

➘ _{detection of structures of about 1 keV} ⇒ very promising results and :

- improvement of acquisition system - understanding of micro-vibrations

peaks at 11.2 keV

Discrimination of the different

contributions of background

for WIMPs detection :

➘ γ rays (natural radioactivity) :

40_K, 214_Bi, 214_Pb, 220_{Ac et} 222_Rn

Î Compton effect >> photoelectric effect : σ_com / σ_pho ~ 10 (at 100 keV)

➘ neutrons (considered as ultimate noise) : Î neutron capture process by the target

nucleus, enhanced after thermalization : σ_cap / σ_ela ~ 10 (at 1 keV)

➘ cosmic muons (energy ~ 2 GeV) : Î energy loss in 3_{He by ionization}

Neutrons separation by capture

process by

_{He nucleus :}

➘ Neutron capture : exoenergetic reaction n + 3_{He p +} 3_{H + 764 keV}

➘ Experiments with Am/Be source ➘ Acquisition time : 4.6 h, à 100 µK

➘ Position of the peak : 650 keV, width 20 keV ⇒ energy resolution of 3 %

rate : 0.5 min-1

➘ _{Shift compared with the expectation value of} 764 keV:

• vortices creation (Kibble mechanism) • UV photons emission

Muons simulation :

➘ Estimate of the expectation counting rate by the single cell prototype : 0.36 min-1

➘ _{Simulation of muons inside the cell with}

GEANT3.21 :

• energy : 2 GeV

• draw of the muons generator :

Muons spectrum :

➘ Counting rate (muons + γ) similar to

estimate ~ 0.36 min-1 _{(due to the fact of a low}

rate of γ : ~ 0.01 min-1₎

➘ _{Shift compared with the simulation :} Î mechanism of UV photons emission ?

67.5 keV

➘ Acquisition time 19 h, at 100 µK ¾ Peak at 45 ± 5 keV

Conclusions :

➘ Two major contributions to background are experimentally shown :

• clear separation of thermal neutrons • detection of cosmic muons

➘ Simulation of muons inside the detector with GEANT3.21

Prospects :

➘ _{Improvement of the analysis method}

➘ _{A better understanding of background caused by}

micro-vibrations is necessary (shape of these peaks would be more symmetric)

➘ Wavelets treatment is investigated

➘ Calibration at low energy : electron conversion

source of 57_{Co with 7, 14, 115 and 129 keV lines is}

considered

Complementarity of MACHe3

with existing projects :

• exclusion limits from Edelweiss, CDMS experiments as well as the DAMA region

• dotted lines indicate projected limits of CRESST and CDMS experiments

Natural radioactivity :

➘ Experiment with a Germanium detector :

⇒ radioactive contamination: 40_K, 214_Bi, 214_{Pb et} 220_Ac

⇒ counting rate

➘ _{GEANT3.21 simulation with :} • Germanium cell

• Helium cell

⇒ ratio of the number of the detected counts ➘ Sources γ : 137Cs and 60Co

• counting rate :

- 137_{Cs : 0.03 s}-1

- 60_{Co : 0.01 s}-1

• in agreement with estimate ➘ γ counting rate without source :

• 0.2 min-1

• with a lead shield (5 cm) : 0.01 min-1

⇒ low sensivity of the detector for γ of natural

χ event rate

in the MACHe3 detector

:

¾ Expression of event rate ( 1st _{approximation) :}

R = σ <v> ( ρ0 / Mχ ) × ( M det / m3He ) thus, R = 1400 × σ (pb) / Mχ (GeV/c2) [kg−1 _day-1] with : <v> ∼ 270 km s-1 ρ0 _{~ 0.3 GeV/c}2 _cm-3 m_3He = 2.81 GeV/c2

¾ For σ ∼ 10−2 _{pb and Mχ ∼ 30 GeV/c}2

Evaluation the

χ relic density

in the Universe :

(logarithmic corrrections are neglected) ➘ For a radiative universe :

Data analysis procedure in

three steps :

- systematic subtraction of the low frequency modulation (polynom of order 5)

- deconvolution

Principle of data analysis :

• raw spectrum f(t) :

with g(t) response function of the wire

h(t) Dirac comb - Fourier transform of f(t) :

- with inverse Fourier transform :

• use of a reference peak for deconvolution :

- at equal temperature, peaks have the same shape: ➘ rising time : 1 s

➘ descending time (at half height) : 10 s (at 100µK)

- descending time : function of the running